This invention relates to an electromagnetic microwave mixer, suitable for operation in the millimeter wave region of the spectrum for mixing a received RF (radio frequency) signal with a reference LO (local oscillator) signal to produce an IF (intermediate frequency) signal and, more particularly, to a mixer producing an image signal in addition to the IF signal and having a microwave filter for reflecting the image signal back into a diode mixing region for further interaction with the LO signal to boost IF power while reducing mixer conversion loss for improved SNR (signal-to-noise ratio).
Low noise millimeter-wave receivers are employed generally in communication and radar systems suitable for electronic warfare, surveillance, radiometry, and instrumentation. It is the practice in the construction of such receivers to employ a mixer without an amplifier at the RF input port of the mixer. Therefore, mixer conversion loss determines sensitivity of the receiver. A reduction of the mixer conversion loss at the front end of the receiver improves the noise figure of the receiver. Therefore, the construction of the mixer is most significant in determining the overall performance of the system.
By way of example, low noise mixer circuits are important in millimeter-wave receiver systems operating in spectral regions, or windows, at 35 and 94 GHz (gigahertz) for use in certain radars, a spectral region at 60 GHz employed for satellite communication, and regions at 140 and 180 GHz employed in radiometric measurements. All of these applications require a receiver with a low noise front end for optimizing performance of the system.
A problem arises in that most of the millimeter-wave mixers developed in recent years have a much higher conversion loss than is desired due essentially to the dissipation of RF power at an image frequency, as well as RF and IF mismatch losses in power and parasitic losses in power. It has been observed that a capturing of power at the image frequency to generate more IF power would produce greater efficiency of a mixer circuit. However, a realization of such power capture at the image frequency has been hampered by losses associated with narrow band filters which might possibly be employed in an RF circuit to retrieve power of the image signal.